The production of polysilicon chunk materials via the decomposition of a gaseous precursor compound on a slim rod substrate is a well-known, widely used process commonly referred to as the “Siemens process.” In a typical Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150° C. with a carrier gas. The trichlorosilane gas decomposes and deposits—silicon onto the heated rods, growing them:2HSiCl3→Si+2HCl+SiCl4 
Silicon harvested from this and similar processes is the polycrystalline silicon. Polycrystalline silicon grown by Siemens process typically has impurity levels of less than 10-9.
In more detail, the Siemens process is a combined decomposition/deposition process that comprises: (1) heating one or more rods or filaments (appropriate substrates) covered by a suitable enclosure to allow high temperature, air-tight operation; (2) feeding a precursor material or compound of desired composition (containing silicon) with no or minimal contamination; (3) further heating the enclosed rods or filaments to a desired temperature under an appropriate environment; (4) decomposing the precursor material preferentially on the heated surface of the rods/filaments to form chunk polysilicon on the substrate or the slim rod; (5) recovering or disposing of byproducts; and (6) recovering the polycrystalline silicon grown slim rods without contaminating them.
In typical Siemens processes and reactors, the reactant gas is fed to the rods from a single port resulting in uneven growth. Such uneven gas distribution over the length of the rod further promotes heavy homogeneous nucleation creating dust. Such uneven growth and homogeneous nucleation promote eventual reactor failure/shut-down. Moreover, the rods within typical Siemens process reactors are not individually isolated. As a result, homogeneous nucleation, lower conversion, higher by-products, and uneven growth on the rods is further promoted by uneven radiant heat between the rods and gas precursor distribution.
Known systems utilizing the Siemens process use at least two power supplies hooked to each reactor system. One or more primary power supply is used for heating and maintaining the temperature of the reactor slim rod (i.e., the rods on which the chuck silicon material is deposited) system for gas decomposition/deposition. A secondary power supply is generally necessary at initiation of heating to overcome the electrical resistance of the silicon rod (greater than about 26,000 volts is needed for a typical for the reactor and also the voltage needed is dependent on the length and diameter of the slim rod assembly used). The necessity for a high voltage power supply significantly increases the cost and safety concerns of operating such known reactors.
Rather than use a very high voltage source, some known reactors use a heating finger introduced into the reaction space and parallel to the deposition rods. To preheat the reactor slim rods for deposition, the heating finger is lowered into the reaction space in the proximity of the slim rods mounted in the reactor. Once the slim rods are at the optimum electrically conductive condition with temperature, the electrical current can be passed through the carrier rods. Then the heating fingers are removed from the reactor and the opening in the metallic enclosure is sealed. Such known reactors present further issues with the purity/integrity of the product, throughput, and establishing and maintaining a seal as well as safety, operational and maintenance issues.
According to known common industrial processes, elemental silicon is obtained in the Siemens type reactor, in the form of cylindrical rods of high purity by decomposing silicon halides from the gas phase at a hot surface of the pure and purified silicon filament, the preferred halides being the chlorides, silicon tetrachloride and trichlorosilane. These compounds become increasingly unstable at temperatures above 800° C. and decompose.
Homogeneous and heterogeneous nucleation processes compete with each other in the reactor. Silicon deposition starts at about 800° C. via heterogeneous reaction and this deposition extends to the melting point of silicon at 1420° C. Since the deposition is beneficial only on the slim rods, the inner walls of the decomposition chamber must not reach temperatures near 800° C. in order to prevent wasteful deposition on the chamber walls.
In known Siemens process reactors, the reactor walls are generally cooled to prevent such wasteful deposition and also to maintain the structural integrity of the assembly. However, cooling the walls consumes additional energy. A further issue with the cooling of the reactor walls is the thermophoretic deposition of powder particles on the cooled reactor walls. Such deposition is generally weak resulting in the multiple re-circulation of the particles in the gas stream. This deposited powder eventually loosens and collapses into the reactor, causing premature failure of the reactor.
The silicon halides used most frequently for the preparation of high purity silicon are silicon tetrachloride and trichlorosilane. These halides will undergo pyrolysis when in contact with hot surfaces and deposit elemental silicon. To obtain reasonable and economical yields, however, an excess of hydrogen gas is added to the silicon halide vapor reaction feed gas. Because of its proportionally higher silicon content per unit weight and comparatively lower deposition temperature (i.e., faster kinetics), trichlorosilane will deposit more silicon than silicon tetrachloride and is therefore the preferred material for the Siemens' process for the preparation of polycrystalline silicon using silicon halide process. However, other silane based precursors can be used in the reaction.
For example, silicon halides with less than three chlorine atoms, such as SiH2Cl2 and SiH3Cl, in particular, deposit much more silicon per mole of silicon halide consumed in the reaction. However, these silicon precursors are not practical because they are not readily available and thus less desirable economically. Also, the yield is not more than 20% (±2%) per each pass through the reactor and the by-product gases are very difficult to handle.
Another approach to improved deposition rates is by using monosilane (aka SiH4 or silane) as the precursor silicon source. The process uses heated mixtures of silane and hydrogen where fast kinetics and lower temperatures assist faster deposition and better conversion than the chlorosilane process. For example, silane offers itself as an effective silicon precursor and, having no chlorine in the molecule, also improves the silicon to hydrogen ratios of silicon reaction gas mixtures. Typically, silane decomposes above 300° C., and more readily above 400° C. forming silicon and hydrogen. That is, the silane decomposition process occurs at much lower temperature than the trichlorosilane decomposition process. Unlike chlorosilane process, the byproducts formed are silane and hydrogen which may be readily recycled.
Typically, the off-gas stream from the monosilane based Siemens reactor contains homogeneously formed polysilicon reaction dust, unconverted reactant gas, by-product gases formed in-situ (disilane, organosilanes etc.) and other impurities present in the reactor and feed gases. Thus, the hydrogen and recovered monosilane streams if re-circulated directly back to the reactor may contaminate the CVD polysilicon process/product and therefore, cannot be reused in the prior art processes. The loss of monosilane and hydrogen in the monosilane based Siemens systems is a further economic drain on the production of polycrystalline silicon due to the raw material loss. Therefore, a system for recovery, purifying and recycling monosilane and hydrogen gas would be very desirable.
What is needed in the art therefore, is an improved Siemens type process for making polysilicon by deposition that recycles silane and/or hydrogen and allows for total utilization of monosilane, and yet produces extremely pure polysilicon in a cost effective and efficient way.